Novartis AG, the Swiss company revolutionized leukemia treatment with Gleevec, a drug that turned a deadly blood cancer into a chronic disease more than a decade ago. Many doctors have since come to believe that the drug, typically taken for life, may heal some patients for good, allowing treatment to be discontinued. The findings highlight one of the company’s biggest challenges: Gleevec, which generated $4.7 billion last year, loses patent protection and sales in 2014.